Hi all. I'm looking for a method/column/mobile phase for analysis of detergent. We have a new dishwasher in the lab and we need to see if all of the detergent/soap has been rinsed off the glassware when the load is finished.
Any help would be appreciated. We have UV/Vis specs, LC's, GC's and IR available for use.

Steve,

I believe you can use a differential refractive index (DRI) detector to measure the residual detergent on your glassware. The DRI has two flow cells in it, one for the solvent (reference cell) and the other for the solvent plus analyte (sample cell). You could use an HPLC pump to flow purified water through the cell and trap it in the reference cell. Then use that same water to rinse your glassware and inject it into the HPLC flow with a manual or auto-injector. The DRI will respond to any difference in the two solvents, including the detergent. No separation column is necessary. You would have to do more work to absolutely identify that the difference in the solvents is actually due to your specific detergent (i.e., molar mass or some other molecular determination). You could also quantitate the amount of residual detergent by measuring the refractive index increment (dn/dc) of your detergent and using that to process the DRI data from your glassware rinse water. Two companies that make DRI detectors are Wyatt Technology and Waters.

Another technique used to quantitate detergents (surfactants) in solution is using surface tension measurements. The more detergent present, the lower the surface tension (think of blowing bubbles as a kid).

Hope this helps.

How about swabbing the glassware followed by water extraction of the swab. The extract would then be spotted on a small TLC plate and the spot charred (H2SO4).You would of course have to establish "passing" specifications.

Steve,

What type of detergent?
If its alkaline it most probably has NaOH as one of its actives. A method I've been using complexes the anionic surfactant of the detergent with a water soluble dye (methylene blue). This complex is then extracted into cloroform and its absorbance is read at 660nm.
A further option would be to blow of the chloroform using nitrogen and resuspend the complex in mobile phase. This can then be injected and the absorbance measured at 660nm.

Hope this helps,

Adrian.

I tried methylene blue years ago with poor results. If you have a coloured detergent, you might try scanning a solution by UV. If there is a helpful chromophore there this can be used to demonstrate absence of detergent down to the detection limit.

Dear Steve,

Do you know whether the surfactants are ionic or non-ionic ?

There is a Triton 100 / Peg 400 mixture (non-ionic) application in the new ThermoQuest Hypersil catalogue. It is unusual because both sets of compound can be analysed in a single run - PEGs using RP chromatography and Triton using NP chromatography. The column is a Hypercarb one. The detection is by evaporative light scattering.

There are also applications for ionic surfactants such as Quadralene 3000, dodecylpyridinium chloride and cetylpyridinium chloride on HyPurity Advance. The detection method is by UV at 254nm.

There is a literature regarding analysis of Alkanesulfonate and alkyl Sulfate Surfactants on anal. chem 64, No 9 p1003.

Low-level anionic surfactants can easily be detected using dimidium bromide-disulphine blue indicator and standardized cationic titrant such as in ASTM 3049-75. Ethoxylated octylphenol or nonylphenol nonionic surfactants can be quantitated using RP-18 columns, H2O/ACN, and UV detection.

We had a similar problem a few years ago and decided against HPLC analysis because of detergents tending to be a blend of many surfactants which suppliers are often reluctant to completely disclose.

We therefore developed a method using a UV scan of rinse solutions. You can make up "standards" using the original detergent. Our method goes down to around 1ppm. However it is subject to interference from other UV absorbing components and so you need to ensure the final rinse is with deionised or distilled water.

Another site has a method using a Total Organic Carbon analyser. Again this is pretty sensitive, but can too be subject to interference from anything else containing organic carbon in the solution

CE is useful for detergent analysis. Low pH for cationic detergent, high pH for anionic like sodium dodecylbenzoate. Indirect UV for SDS etc. Many detergents are caustic and the Na+ or K+ can be monitored by indirect UV. Have a look at J.Liquid Chromatography and Rel. Technologies, 21 1998 1093-1105. For more CE info take a look at http://www.ceandcec.com